(1) Field of the Invention
The present invention relates to a light beam scanning system for scanning an image with a light beam emitted from a light source to record or read the image. More particularly, the invention relates to a light beam scanning system capable of detecting focus displacement of a light beam on a scanning plane.
(2) Description of the Related Art
Where a light beam scanning system is used to record or read an image, it is desirable from the viewpoint of resolution that the light beam has a minimal diameter (a beam diameter at a focal point of the light beam, which is hereinafter referred to as a focus diameter) on the scanning plane.
In an ordinary light beam scanning system, therefore, a photosensitive film is set to the scanning plane prior to an actual scan. The film is then scanned by a light beam for printing of an image. A line width in the printed image provides the basis for determining whether the beam is focused on the scanning plane or not. In the event of a focus displacement, adjustment is effected between a position on the scanning plane and the focal point of the light beam.
According to the above method, the film must undergo a developing process in order to detect the focal point of the light beam, with the developed image subjected to observation by a skilled person. Thus, this method involves many steps, resulting not only in high cost but in low efficiency.
Under the circumstances, light beam scanning systems having a mechanism for detecting a light beam diameter have been proposed as in Japanese Patent Publications Nos. 1985-29087 and 1985-9243, for example. These systems will be described below.
(A) First, the system disclosed in Patent Publication No. 1985-29087 will be described with reference to FIG. 1.
In this system, a light beam emitted from a laser source 100 travels via a reflecting mirror 101 to a beam expander lens 102 and a collimator lens 103 to become a collimated beam. The collimated beam then travels via a scanning mirror 104 to a scanning lens 105 to be focused on a scanning plane 107. The scanning mirror 104 is oscillatable to cause the light beam to scan the scanning plane 107.
For detecting a light beam diameter occurring on the scanning plane 107, a half mirror 106 is disposed between the scanning lens 105 and scanning plane 107 to branch out part of the light beam. A lattice plate 109 is disposed on a virtual scanning plane 108 where the branched light beam focuses.
The lattice plate 109 has a lattice pattern with light transmitting sections and light shielding sections arranged alternately at intervals approximately corresponding to a focus diameter of the light beam. As illustrated, the lattice plate 109 is disposed at a predetermined angle to the virtual scanning plane 108.
Light transmitted through the lattice plate 109 is detected by an optical detector 110. At this time, a maximum quantity of light impinges on the detector 110 from the light transmitting section corresponding to the focal point of the light beam, which raises a detection signal level to a peak. If a central point A of the lattice plate 109 provides a detection signal in the highest level, the light beam has the focal point on the virtual scanning plane 108, which means that the light beam is also focused on the actual scanning plane 107. If the focus of the light beam is displaced from the scanning plane 107, the light transmitted through the lattice plate 109 provides a detection signal having a peak appearing in a position displaced from the central point A. Thus, the peak point of this detection signal is used to detect the focal point of the light beam on the scanning plane 107 and an amount of displacement between the position of the scanning plane 107 and the focal point of the light beam.
(B) The system disclosed in Patent Publication No. 1985-9243 will be described next with reference to FIG. 2.
In this system, a light beam emitted from a laser source 111 travels through a light modulator 112 and a beam expander 113 to a rotating polygon mirror 114 and then to a scanning lens (f.theta. lens) 115 to be focused on a scanning plane 116. The polygon mirror 114 is rotated to cause the light beam to scan the scanning plane 116.
For detecting a light beam diameter occurring on the scanning plane 116, a knife edge 118 is disposed on an extension from a scan line on the scanning plane 116. A photodetector 117 is disposed behind the knife edge 118.
The photodetector 117 detects a quantity of a light beam impinging on it through the knife edge 118, and generates a signal corresponding to a light beam diameter. That is, when the light beam has a large diameter, a gradually increasing quantity of light impinges on the photodetector 117 through the knife edge 118. When the light beam has a small diameter, a rapidly increasing quantity of light impinges on the photodetector 117.
In this way, the focus of the light beam is detected, and adjustment is made between the focal point of the light beam and the position of the scanning plane, on the basis of variations in the detection signal outputted from the photodetector 117, which variations are responsive to the light beam diameter.
The conventional scanning systems with the foregoing constructions have the following disadvantage.
The system described under (A) above detects a quantity of light transmitted through the inclined lattice plate 109 and determines a focal point of the light beam from a peak position of the detection signal. This system does not detect the focal point throughout a scanning width of the light beam. Similarly, the system described under (B) can detect the focus of the light beam only at one point on the extension from a scan line.
However, the light beam may have different diameters at different points on the same scan line, which is due to distortions occurring with optical elements such as a scanning lens and polygon mirror or deviations from correct positions. It is, therefore, inadequate to detect the focus of the light beam only at a single point as in the conventional systems.
The conventional systems are incapable of detecting the light beam out of focus on a scan line not corresponding to the point of detection. As a result, an image is recorded or read with a low degree of resolution, i.e. leading to the problem of a blurred image.